The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics of light weight, high strength, toughness, thermal resistance, and ability to be formed and shaped can be used to great advantage. Such components are used, for example, in aeronautical, aerospace, satellite, high performance recreational products, marine, and other applications.
Typically, such components consist of reinforcement materials embedded in a matrix material. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid, polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. These materials are often fabricated into fibers and used as reinforcing fibers, or the fibers are formed into yarns which are used as reinforcing yarns in the component.
Through the use of such reinforcement materials, which ultimately become a constituent element of a completed component, the desirable characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The typical constituent reinforcement materials may be woven, knitted or otherwise oriented into desired configurations for reinforcement preforms. In many cases, particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected. Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components.
After the desired reinforcement preform has been constructed, a resin or matrix material may be introduced to and into the preform, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the interstitial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, bismaleimide, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical thermal or other properties. Typically, however, they will not be of the same materials or have comparable physical, chemical, thermal, or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforced preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified masses of the matrix material normally are very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to, and borne by, the constituent material of the reinforcement preform. Any break or discontinuity in the reinforcement preform limits the ability of the preform to transfer and bear the stress applied to the finished component.
In certain applications, three dimensional (3D) woven composite structures are desired as primary load carrying members. One useful shape of a preform for such members is generally referred to as a “Pi” preform, so called because it resembles the Greek letter pi (Π) in an axial view. Other useful preforms may have different cross sectional shapes, such as T or L for example. Fiber preforms with specific structural shapes can be woven on a conventional shuttle loom, and several existing patents describe the method of weaving such structures (U.S. Pat. Nos. 6,446,675; 6,712,099; 7,712,488, for example).
One of the drawbacks of the use of these preforms is that they cannot be formed into corner fittings without darting the upstanding leg or legs according to conventional methods. By darting the legs, the continuity of the reinforcing material is lost through the corner, removing or reducing the primary load path through the corner. For example, if the preform shapes disclosed above are formed into a corner fitting, excess material of the leg(s) parallel to the plane of the bend (i.e., the inside of the bend) will accumulate and buckle at the inside of the corner.
To maintain the structural integrity of the corner fitting preform, in many cases the addition of reinforcements is required at the cut or cuts, and around the corner itself. The reinforcement is often in the form of sheets or plates of material, typically additional woven material. The additional reinforcement creates a localized increase in thickness and weight of the corner fitting preform. The reinforcement may create a localized weight concentration in the reinforced corner itself.
One such reinforcement for a darted preform is disclosed in U.S. Patent Application Publication No 2011-0111664 A1 the entire content of which is incorporated herein by reference. The reinforcement for a darted preform provided by this reference is in the form of a steered woven fabric which fills the space between the upstanding legs of a Pi preform and is steered by weaving to follow the curvature facilitated by the darting of the legs. After the steered woven fabric is in place, the preform is further processed to form a reinforced composite structure.
Other known methods may require mechanical fasteners, for example, bolts or rivets, to affix the reinforcement to the preform at the corner. However, the use of metal bolts or rivets at the interface of such components is often unacceptable because such fasteners require through holes which further compromise the integrity of the composite structure. Detrimentally, fasteners add weight and introduce different coefficients of thermal expansion as between such elements and the surrounding material.
Prior art methods have not adequately addressed the need for 3D woven preforms able to be formed into corner fittings without the addition of reinforcing materials and the resultant increase in localized thickness and additional weight. The present invention addresses the shortcomings of the prior art by providing a 3D woven preform that can be formed into a corner fitting with a lap joint without the need for additional reinforcement and associated increase in localized thickness and additional weight.